Sitosterol and campesterol are abundant naturally occurring sterols, which have long been recognized as valuable intermediates for steroid hormone production. Several economical microbiological processes are now available for the transformations of these sterols into useful steroid hormones via the selective leavage of the hydrocarbon side chain without fission of the steroid nucleus. The objective of this proposal is to define the intermediates, the reaction sequence and the enzymatic mechanisms involved during the initial stages of the degradation of these branched hydrocarbon side chains. The availability of this basic knowledge should greatly aid in the improvement of the existing microbiological processes for steroid hormone production from these sterols. Moreover, mutants may be selected for the accumulation of other valuable intermediates such as C24 acids and C26 alcohols for the synthesis of bile acids, corticosteroids and 25-hydroxy-vitamine D derivatives. In addition, several novel enzymatic reactions need to be studied in terms of their coenzyme requirements and reaction mechanisms.